1. Field of the Invention
The present invention relates to a photodetector used in a form incorporated in an optical pickup device that performs an operation of reading out a signal recorded in an optical disc or recording a signal in an optical disc.
2. Description of the Related Art
There is widely used an optical disc device capable of applying the laser light emitted from an optical pickup device to a signal recording layer of an optical disc thereby performing an operation of reading out and recording a signal.
While the optical disc device for use with the optical disc such as CD and DVD is generally in wide use, recently the optical disc device has been developed, for use with the optical disc with improved recording density, namely, the optical disc of Blu-ray standard or HD-DVD (High Definition Digital Versatile Disc) standard.
An infrared light with a wavelength of 780 nm is used as the laser light for the operation of reading out a signal recorded in the optical disc of the CD standard, and a red light with a wavelength of 650 nm is used as the laser light for the operation of reading out a signal recorded from the optical disc of the DVD standard.
As compared to the above optical discs of the CD standard and the DVD standard, the laser light with a shorter wavelength, e.g., a blue light with a wavelength of 405 nm is used as the laser light for the operation of reading out a signal recorded in the optical disc of the Blu-ray standard or the HD-DVD standard.
There is provided a protective layer with a thickness of 0.1 mm on the top surface of the signal recording layer of the optical disc of the Blu-ray standard, and the numerical aperture of an objective lens is set at 0.85 for use in performing the operation of reading out a signal from the signal recording layer.
On the other hand, there is provided a protective layer with a thickness of 0.6 mm on the top surface of the signal recording layer of the optical disc: of the HD-DVD standard, and the numerical aperture of the objective lens is set at 0.65 for use in performing the operation of reading out a signal from the signal recording layer.
As described above, since the blue light with a wavelength of 405 nm can be used as the laser light for performing the operation of reading out signals recorded in the optical discs of the Blu-ray standard and the HD-DVD standard, there can be produced the optical pickup device capable of performing the operation of reading out signals from the optical discs of these two standards, by using a laser diode in common for both optical discs.
However, in order to read out signals from both optical discs, the numerical aperture needs to be changed corresponding to each of the optical discs, since both optical discs are largely different in the positions of the signal recording layers and in the numerical aperture required of the objective lens. There has been developed the optical pickup device capable of performing the above operation (see Japanese Patent Application Laid-Open Publication No. 2006-172605).
The optical pickup device conforming to the optical disc standards where the recording density is to be improved, requires high optical properties to enhance signal recording quality in accordance with the improvement of the recording density.
The optical pickup device is configured to be able to control a driving current that is supplied to the laser diode, so that the laser outputs can be obtained, which is suitable for reading out a signal recorded in the optical disc or which is suitable for recording a signal in the optical disc.
The optical pickup device is also configured to be able to perform: a control of focusing the laser light emitted from the optical pickup device on the signal recording layer of the optical disc, namely, a focusing control operation; and a control of causing the focusing spot of the laser light to follow a signal track, namely, a tracking control operation.
The photodetector is incorporated in the optical pickup device as a means for the laser output control operation, the focusing control operation, and the tracking control operation, described above.
The photodetector incorporated in the optical pickup device will be described with reference to FIG. 4. FIG. 4 is a cross-sectional view of the photodetector. A photodiode 2 that outputs a signal corresponding to light amount of the received laser light is fixed on a substrate 1, and also the photodiode 2 is covered by a translucent resin member 3 having high transmittance. While a signal generated by the photodiode 2 is taken out through a wire bonding, such a wire is fixed and protected by the translucent resin member 3.
The laser light used for the DVD-type optical disc, namely, the red laser light, has a wavelength of 650 nm, while the laser light used for the optical disc of the Blu-ray or HD-DVD standard, namely, the blue laser light, has a shorter wavelength of 40.5 nm.
The relationship between the energy and the wavelength of the laser light is expressed as E=hν=h×c/λ where E is energy, h is Planck's constant, ν is the number of vibrations, c is light, speed, and λ is wavelength. As is apparent from the above relationship, the laser light has the characteristics that the shorter the wavelength of the laser light becomes, the higher the light energy becomes.
In the optical pickup device incorporating the laser diode that emits the laser light, while the laser output control operation is performed utilizing a signal obtained from the photodetector, the operation of detecting light amount of the laser light is performed by the photodetector based on the laser light emitted through the translucent resin member 3 that covers the photodiode 2.
In view of performance and cost, epoxy resin is generally used as a material of the translucent resin member 3. In the case of using resins, however, since the transmittance continues to fall due to a photochemical reaction, the laser output for reproducing and the laser output for recording change. Especially in the case of performing the recording operation, since the laser output control operation is required to be performed precisely according to the recording characteristics of the optical disc, there is a problem that the recording operation can not be performed precisely when the detecting operation is not precisely performed by the photodetector due to the fall of the transmittance.
Although there is a method of covering the photodiode 2 with glass for solving the above problem, such a method causes a problem that the price of the optical pickup device increases due to a high cost of glass. When the photodetector having the photodiode 2 covered with the above translucent resin member 3 is employed for the optical pickup device that uses the blue laser light, since light energy of the laser light is high, the photochemical reaction of the resin is strong, which causes the transmittance of light receiving portion 3a to be fallen. As a result, such a problem occurs that the performance of the optical pickup device can not be maintained for a long time.